Considering Environmental Conditions – Density Altitude

One of the biggest hurdles I had to overcome as I became involved in long range shooting, was the ability to easily and accurately consider changes in the environmental conditions to allow a predictable change in DOPE.

Changes in the environmental conditions, mainly due to shifts in Temperature and Humidity, occur during the course of a day, over the duration of the calendar year and also with moving to different shooting locations, for example the difference in altitude or elevation between the local range and a hunting trip. The wind is another changing condition because it seems to never rest, especially at the most inopportune times. Dealing with wind will be discussed at a later date, but other changing conditions, albeit at a much slower rate, temperature and humidity will tackled in this article.

Background

As I started to shoot beyond 500 yards for the first time, I noticed that the bullet drop I had used at the last range session no longer worked. This caused me some problems because I wasn’t trying to use the exact environmental conditions, just the bullet drop from the last trip to the range. I would therefore have to measure all the new conditions and re-run my ballistics program. What I needed was a simple way to predict how the bullet would be affected by the new conditions, or the conditions I was experiencing real time.

Density Altitude

After getting some guidance from an experienced competitive shooter, I conducted careful research and found that temperature, humidity and elevation were all directly proportional to each other, which means as one factor changes there’s a knock on affect with the others. As an engineer, I desired a single formula to cover all of these factors. Measuring multiple conditions and inputting them individually didn’t appeal to me. I wanted something easier where I could pre-print my data cards and just select the one that fitted the conditions on the day.

My quest for a single formula led me to the Density Altitude formula commonly used by pilots to estimate the drag and therefore helps determine the available power on any given day by considering the elevation and the temperature at any given time. What could be easier? For my application, I could research the elevation of the local range and record for future use. The only change I would have to consider would be the temperature on the day, reference the chart (or buy a Kestrel that provides the density altitude number) and input to the ballistics program. This would give me the new dope, dial it and shoot. I like to understand the chart first and the theory behind it, rather than just rely on a battery operated device. I also carry a laminated copy of the chart to use in emergencies.

See Figure 1 below for a standard Density Altitude chart I use for reference –

Figure 1

The horizontal axis provides the temperature selection and the vertical axis provides the elevation selection for the location. The diagonal lines represent the equivalent Density Altitude to be used when running a ballistics program to predict the new bullet drop and wind drift.

As an example, lets use the elevation of my local range – 5,000ft altitude. When I arrive at the range in winter, the temperature is 40 oF. Using these two pieces of data, I plot them on the chart. See figure 2 below.

Figure 2

In reviewing the chart, the lines plotted for elevation and temperature intersect and have been denoted by the use of a red circle. When this intersect point is viewed against the diagonal lines, it can be noticed that the conditions equivalent to 5,000ft Density Altitude environmental conditions. This line is denoted by the dotted red line.

If I go to my local range in summer and the temperature was 90 oF, I would move the vertical line over and read the new intersect point. See figure 3 below.

Now the equivalent Density Altitude is 8000ft.

Figure 3

Also included is my first hunt of the year at 10,000ft altitude and 70 oF weather (13,000ft DA equivalent). See figure 4 below.

Figure 4

To allow a comparison, we apply a maximum range of 1000 yards with a 308 Winchester, 168 gr Winchester Match @ 2,680 fps. The difference between 5,00ft DA, 8,000ft and 13,000ft DA in terms of bullet drop and wind drift can be seen in Figure 5 below.

Figure 5

The difference in bullet drop between 5,000ft DA and 13,000ft DA is 53.5 inches and wind drift is 25.9 inches. That’s a big change between the local range in the winter and the fall hunting season in the mountains. This is an extreme example, but it demonstrates how the factors behave in relation to each other.

What about Texas (1000ft above sea level) from summer (95oF) to winter (40oF)? See figure 6 below. Same caliber and velocity.

Figure 6

In reviewing the data presented above, the use of the Density Altitude formula is simple and easy to follow. Without a Kestrel or similar, its easy to make changes on the fly is you research the elevation of your shooting location and take the temperature before you start shooting. I have found the thermometer in my truck to be accurate enough to be used when I’m in the mountains.

For shooting competitions or planning hunts, I can pre-plan and create my laminated data cards by looking at the seasonal weather extremes for the location in question. For example, the Competition Dynamics – Rocky Mountain Steel Quest was held this year in Douglas WY during April. I found the elevation of the location was ~5,000ft and the seasonal extremes were 30 oF through 75 oF. This allowed me to pre-print my data cards from 4000ft DA to 7,000ft DA in 1,000ft intervals. See DA spread in Figure 7 below from 40 oF thru 100 oF, at 5,000ft elevation.

Figure 7

Now that we have established the benefits of using Density Altitude, a simple $20 app can be downloaded to your smart phone and used with great ease.

Ballistics App

I typically use the iphone App – Ballistic AE. It works well and I’m comfortable enough with it now that I can easily spot mistakes or bad info. To start using Density Altitude, download the app, go to the general phone settings and find the App. Scroll through the settings until you find the setting for Atmosphere, then format, and finally select the Density Altitude format option. See figure 8 below.

Figure 8

At this time, the phone must be rebooted. Switch off and switch back on for the changes in format to take affect. If not, you won’t see any changes until the next time your phone dies. Follow the instructions to input your load and an accurate data card is produced.

Important Note – To ensure the data card is valid, verify the true velocity of your load. This is essential! If you don’t know the true velocity of your load, follow the steps below –

1. Use an estimate ballpark velocity. Example – 2640 fps.
2. Create a data card using the estimated velocity. Example – 2,640 fps.
3. Use the correct density altitude for the location and current temperature. Example – 8,000ft DA.
4. Assuming you have a scope that allows elevation to be dialed, dial for atleast 750 yards. Example – 750yds = 5.9 MIL.
5. Shoot and watch for the splash. Using the reticle on your scope, measure the distance of the splash from where you aimed. Add or subtract the error value to your original elevation change. Example – Shot was high by 0.5 MIL. Actual value to be dialed = 5.4 MIL.
6. Knowing the error value, adjust the velocity in the app and re-run. Keep adjusting the velocity until the elevation to be dialed matches what happened at the range. 5.4 MIL = 2,780 fps.
7. Dial into 500 yards and check. Dial out to 1,000 yards and check.

Assuming all the data inputted has been accurate, you now have the true muzzle velocity without using a chronograph. Remember garbage in = garbage out. Be methodical and enter the data properly. If you don’t have a certain value like the bullet BC or sight height about bore, research it and enter correctly.

The app requests all the required data in a logical fashion and it becomes easy to use.